if it's nice go to either peckham rye, dulwich park or crystal palace park for the fab dinosaurs. kids that age will love them. Or get the train to london bridge- a treat in itself for that age group and just walk either way along the south bank for a bit. then the parents can go to the tate or the design museum. London aquarium is also great. Or.. the science museum basement 'hands on' space is great, aimed at exactly this age group with a great water play thing, blocks to build and knock down, dressing up.............and more and then you can check out the rest of the science museum too. also great cos you can take your own sarnies and eat them in indoor picnic area if needs be. or if you are feeling flash and the kids have been entertained then have lunch at the V&A cafe which is by far the best place to eat out of those 3 musuems. buy a sticker book from the science museum to keep the kids quiet while you eat lunch. Or the horniman is great if its raining and you dont want to go too far. if you are venturing into london proper take snacks- take extra clothes, take some colouring or a little book or something and take a buggy for the small one and double your normal journey time when calculating how long stuff takes.